The Fossil Rich Region of Tunica Hills, Louisiana

The Tunica Hills region is located in the geographical corner between southwestern Mississippi and southeastern Louisiana.  Ice Age climate variations instead of tectonic processes created these uplands.  During times of glacial expansion the Laurentide glacier scraped bedrock in the upper midwest, grinding it down into what geologists actually call “rock flour.”  During interstadials meltwater pulses carried this rock flour down the Mississippi River.  The warm climatic phases that caused meltwater pulses always preceded changes to cold arid conditions.  The decrease in precipitation led to a greatly reduced flow in the Mississippi River, and massive amounts of river sediment became exposed to the air in giant sandbars and dunes.  The windy environment of this glacial stage blew this river sediment, which consisted largely of rock flour (also known as loess), to the east where it settled in big hills.  So the Tunica Hills are actually great big piles of midwestern rock dust.  Later, streams (known as bayous in Louisiana) eroded through the hills and cut through fossil deposits of different ages, washing Miocene, Pliocene, and Pleistocene fossils into the water for today’s fossil hunters to find.

Ravine in the Tunica Hills.  The walls are made of midwestern rock dust blown here during the Ice Age.  From google images.

The loess buried and preserved lots of paleobotanical evidence from the last Ice Age–not only pollen but macrofossils such as logs, cones, and leaves.  The age of the pollen and fossil plants gives us a continuous record from 24,500 BP-17,700 BP, the height of the Last Glacial Maximum.  (The Laurentide Glacier reached its southernmost extent 18,000 BP, but I consider the whole time span of ~28,000 BP-~15,000 BP to be the LGM because that’s when Wisconsinian Ice Age climate was the coldest.  However, there were several weak and brief interstadials within this timespan.)  Scientists found evidence of a forest dominated by spruce but with hardwoods such as oak, beech, elm, hickory, walnut, and maple present.  Spruce made up 40%-70% of the pollen, pine 6%-17%, oak 3%-10%, and other hardwoods <2%.  Grass and composites such as aster, daisies, and sunflowers were also well represented, indicating widespread meadows.  Dwarf mistletoe, an interesting parasite that grows on spruce trees, was abundant.

At first scientists misinterpeted this data because they catalogued the spruce fossils as belonging to an extant boreal species–white spruce.  But in 1999 scientists identified the species they were finding as an extinct temperate species they’ve named Critchfield’s spruce.  Formerly, scientist believed that during the LGM the Tunica Hills region was comparable in climate to the modern day Great Lakes region.  Critchfield’s spruce was likely adapted to warmer climatic conditions than white spruce, nullifying this previous assumption.  Instead, climate in Louisiana then was probably only slightly cooler than today.

Conifers and grasses grew better in an Ice Age atmosphere with lower CO2 levels than did broad-leaved trees.   This probably accounts for why Critchfield’s spruce became a dominant tree over hardwoods.   It was lower atmospheric CO2 rather than lower temperatures that facilitated the growth of this forest.   This unique ecotone of dominant spruce with mixed hardwoods and meadows stretched all the way from Tunica Hills east to Kentucky, Tennessee, and western Georgia.  Critchfield’s spruce fossils have also been found in 2 different localites in western Georgia and 1 in Tennessee.  Eastern Georgia and South Carolina had a somewhat different landscape with more pine and oak, though spruce was present.  West of Tunica Hills an open oak (probably bur oak) and cedar savannah prevailed–grasses and composites made up 20%-50% of the pollen, oaks 10%-25%, and spruce 1%-15%.

Today, Tunica Hills supports an unusual forest for the region.  Beech, magnolia, and holly are dominant but many species occur including laurel oak, water oak, osage orange, hackberry, maple, silver bell, paw paw, and river cane.  Cool ravines provide relief from the hot southern sun, so trees with northern affinities grow next to warm climate trees.  A disjunct population of eastern chipmunks resides here.

The Miocene deposit is located geologically in the Pascagoula Formation.  It’s believed to have been an estuary then.  The Miocene (25 million BP-5 million BP) could be known as either the age of the horse or the age of the rhino because they were the 2 most common large mammal species then.

Hipparion–a 3 toed gazelle horse common during the Miocene.  Miocene Louisiana was mostly tropical.

Naturalists excavated specimens of both the hippo-like rhino (Teleoceras) and the hornless rhino (Aphelops) from Tunica Hills.  At least 5 different species of horses galloped the region then–Astrohippus, Cormohipparion, Hipparion, Neohipparion, and Nannihippus aztecus.  They were all 3-toed gazelle-horses.  The cavalcade of Miocene ungulates also included peccaries, tapirs, an extinct species of pronghorn antelope, and the odd synthetocorus

A synthetocorus. What an odd hooved animal.  Note the bone on its nose.

Mammut mathewii roamed what was to become Louisiana then.  It was an early form of mastodon.  Giant tortoise, alligator, snake, and fish remains were collected as well.

Some of these species survived to early in the Pliocene but among the 3-toed horses, only Nannihippus aztecus survived to the end of the Pliocene, sharing the range with the 1 toed modern genera of horses which then included the American zebra.

A list of Pleistocene fossils collected from the Tunica Hills region consists of the familiar animals discussed on this blog–Jefferson’s ground sloth, Harlan’s ground sloth, horse, saber-tooth, giant armadilloes, and grouse.  A glyptodont fossil is the most recent addition to the Tunica Hills fossil list.  This year scientists identified glyptodont scutes and a partial rib from a Tunica Hills creek.  This is the first find of this species between Florida and Texas.

Reconstructed glyptodont displayed at the University of Florida museum.  I don’t know if this is a replica or the real bones.

Scientists refer to fossils found in the bayou as “float” though they don’t literally float but are usually recovered by sifting the bottom.  The age of the fossils in Tunica Hills can’t be determined based on stratigraphy because streams eroded them from different age strata, but scientist have figured out how to used an interesting method to distinguish whether a fossil is late Miocene/early Pliocene or Pleistocene.

Some of the highlighted elements (known as rare earth elements) are the ones that can be used to determine which age a fossil belongs to.  The ratio of these elements varies in the environment over time.

Scientists use knowledge of rare earth elements to determine the relative age of different fossils.  One collegel student, Lindsey Yann, conducted a study for her Masters thesis comparing the ratios of rare earth elements in different fossils found in the Tunica Hills bayous.  Rare earth elements (a bit of a misnomer because they’re not particularly rare) include elements on the periodic table numbered 57-71.  They occur in groundwater in certain fixed ratios.  Animals absorb ground water by ingestion and then for thousands of years after they die their bones continue to become saturated with it.  Eventually, the bone reaches a saturation poiunt and won’t take in any more.  The ratio of rare earth elements in that particular fossil becomes fixed.  However, over millions of years the ratios of rare earth elements in ground water changes.  So an animal that lived 5 million years ago will have a different ratio of rare earth elements than an animal that lived 25,000 years ago.

References:

Jackson, Stephen; and Charles Givens

“Late Wisconsinian Vegetation and Environment of the Tunica Hills Region, Louisiana and Mississippi”

Quaternary Research 41 1994 pp. 306-325

Schiebout, J.; et. al.

“Miocene Vertebrate Fossils Recovered from the Pascagoula Formation in southeastern Louisiana”

Geology Paper

Schiebout, J.

“Fossil Evidence of Glyptotherium C.F. floridanus in the Pleistocene of Lousiana”

GSA paper March 2011

Yann, Lindsey

“Rare Earth Elements as an Investigative Tool into the source, age, and ecology of Late Miocene to Late Pleistocene Fossils from the Tunica Hills, Louisiana”

Masters thesis for LSU

http://etd.lsu.edu/docs/available/etd-06292010-133339/unrestricted/YannThesis.pdf

Tags: Critchfield's spruce, Ice Age forests, Louisiana fossils, rare earth elements, Tunica Hills